1. Technical Field
The present disclosure relates to an aluminum alloy wire rod used as a conductor of an electric wiring structure, an aluminum alloy stranded wire, a coated wire, a wire harness, and a method of manufacturing an aluminum alloy wire, and particularly relates to an aluminum alloy wire rod that has an improved impact resistance and bending fatigue resistance while ensuring strength, elongation and conductivity equivalent to the related art products, even when used as an extra fine wire having a wire diameter of less than or equal to 0.5 mm.
2. Background
In the related art, a so-called wire harness has been used as an electric wiring structure for transportation vehicles such as automobiles, trains, and aircrafts, or an electric wiring structure for industrial robots. The wire harness is a member including electric wires each having a conductor made of copper or copper alloy and fitted with terminals (connectors) made of copper or copper alloy (e.g., brass). With recent rapid advancements in performances and functions of automobiles, various electrical devices and control devices installed in vehicles tend to increase in number and electric wiring structures used for devices also tends to increase in number. On the other hand, for environmental friendliness, lightweighting of transportation vehicles is strongly desired for improving fuel efficiency of transportation vehicles such as automobiles.
As one of the measures for achieving lightweighting of transportation vehicles, there have been, for example, continuous efforts in the studies of using aluminum or aluminum alloys as a conductor of an electric wiring structure, which is more lightweight, instead of conventionally used copper or copper alloys. Since aluminum has a specific gravity of about one-third of a specific gravity of copper and has a conductivity of about two-thirds of a conductivity of copper (in a case where pure copper is a standard for 100% IACS, pure aluminum has approximately 66% IACS), an aluminum conductor wire rod needs to have a cross sectional area of approximately 1.5 times greater than that of a copper conductor wire rod to allow the same electric current as the electric current flowing through the copper conductor wire rod to flow through the pure aluminum conductor wire rod. Even an aluminum conductor wire rod having an increased cross section as described above is used, using an aluminum conductor wire rod is advantageous from the viewpoint of lightweighting, since an aluminum conductor wire rod has a mass of about half the mass of a pure copper conductor wire rod. Note that, “% IACS” represents a conductivity when a resistivity 1.7241×10−8 Ωm of International Annealed Copper Standard is taken as 100% IACS.
However, it is known that pure aluminum wire rods, typically an aluminum alloy wire rod for transmission lines (JIS (Japanese Industrial Standard) A1060 and A1070), is generally poor in its durability to tension, resistance to impact, and bending characteristics. Therefore, for example, it cannot withstand a load abruptly applied by an operator or an industrial device while being installed to a car body, a tension at a crimp portion of a connecting portion between an electric wire and a terminal, and a cyclic stress loaded at a bending portion such as a door portion. On the other hand, an alloyed material containing various additive elements added thereto is capable of achieving an increased tensile strength, but a conductivity may decrease due to a solution phenomenon of the additive elements into aluminum, and because of excessive intermetallic compounds formed in aluminum, a wire break due to the intermetallic compounds may occur during wire drawing. Therefore, it is essential to limit or select additive elements to provide sufficient elongation characteristics to prevent a wire break, and it is further necessary to improve impact resistance and bending characteristics while ensuring a conductivity and a tensile strength equivalent to those of the related art.
For example, aluminum alloy wire rods containing Mg and Si are known as high strength aluminum alloy wire rods. A typical example of this aluminum alloy wire rod is a 6xxx series aluminum alloy (Al—Mg—Si based alloy) wire rod. Generally, the strength of the 6xxx series aluminum alloy wire rod can be increased by applying a solution treatment and an aging treatment. However, when manufacturing an extra fine wire such as a wire having a wire size of less than or equal to 0.5 mm using a 6xxx series aluminum alloy wire rod, although the strength can be increased by applying a solution heat treatment and an ageing treatment, the elongation tends to be insufficient.
For example, Japanese Laid-Open Patent Publication No. 2012-229485 discloses a conventional 6xxx series aluminum alloy wire used for an electric wiring structure of the transportation vehicle. An aluminum alloy wire disclosed in Japanese Laid-Open Patent Publication No. 2012-229485 is an extra fine wire that can provide an aluminum alloy wire having a high strength and a high conductivity, as well as an improved elongation. Also, Japanese Laid-Open Patent Publication No. 2012-229485 discloses that sufficient elongation results in improved bending characteristics. However, for example, it is neither disclosed nor suggested to use an aluminum alloy wire as a wire harness attached to a door portion, and there is no disclosure or suggestion about impact resistance or bending fatigue resistance under a severe operating environment in which a fatigue fracture is likely to occur due to repeated bending stresses exerted by opening and closing of the door.
The present disclosure is related to providing an aluminum alloy wire rod used as a wire rod of an electric wiring structure, an aluminum alloy stranded wire, a coated wire, a wire harness, and a method of manufacturing an aluminum alloy wire rod that has an improved impact resistance and bending fatigue resistance while ensuring strength, elongation and conductivity equivalent to those of a product of the related art (aluminum alloy wire disclosed in Japanese Laid-Open Patent Publication No. 2012-229485), even when it is a prerequisite to use an aluminum alloy containing Mg and Si and by making a microstructure appropriate, and particularly when used as an extra fine wire having a strand diameter of less than or equal to 0.5 mm.
The present inventors have observed a microstructure of the aluminum alloy wire of the related art containing Mg and Si, and found that a zone free of precipitates consisting of a compound of, for example, Mg, Si, Fe, Ti, B, Cu, Ag, Au, Mn, Cr, Zr, Hf, V, Sc, Co and Ni, which are alloy elements added into aluminum, is formed at a portion of a grain that is inside the grain and located in the vicinity of a grain boundary. Such region is a so-called precipitate free zone (PFZ: Precipitate Free Zone). Then, the present inventors have carried out assiduous studies under the assumption that such PFZ has a composition substantially equivalent to that of a pure aluminum and thus has a property equivalent to that of a pure aluminum, resulting in a decrease in a tensile strength, elongation, impact resistance and bending fatigue resistance.
The present inventors have prepared various types of aluminum alloy wires with various widths of precipitate free zone (PFZ) formed at a portion of a grain that is inside the grain and located in the vicinity of the grain boundary by controlling a component composition and a manufacturing process, and carried out a comparison. As a result, it was found that, in a case where the width of the precipitate free zone (PFZ) is made narrow to a certain extent, an improved impact resistance and bending fatigue resistance can be achieved while ensuring strength, elongation and conductivity equivalent to those of a product of the related art (aluminum alloy wire disclosed in Japanese Laid-Open Patent Publication No. 2012-229485).
Further, the present inventors have found that since a portion which is a precipitate free zone (PFZ) has a soft and easily deformable structure, and a portion where precipitates exist (precipitate zone) has a structure which is comparatively rigid and difficult to deform, a grain boundary strength and an elongation decrease. Accordingly, the present inventors have also found that reducing the width of the precipitate free zone (PFZ) is preferable in improving tensile strength and elongation (uniform elongation), and contrived the present disclosure.
Note that when an aluminum alloy wire rod is non-uniformly deformed, a local elongation occurs and a cross section area of the aluminum alloy wire becomes locally small, and as a result, a conductor resistance increases and there is a risk that an electric wire may give off smoke due to joule heat emitted by the aluminum alloy wire itself. This tendency becomes particularly noticeable when such an aluminum alloy wire is used as an extra fine wire having a strand diameter of less than or equal to 0.5 mm, since a contribution ratio of the PFZ width to the cross sectional area becomes higher.
Further, in Japanese Laid-Open Patent Publication No. 2003-105473, which was also filed by the present applicant and which is an unexamined application laid open to public, the applicant has already proposed an aluminum alloy sheet having an improved bending workability and draw-molding by reducing the width of PFZ. However, in the technique disclosed in Japanese Laid-Open Patent Publication No. 2003-105473, it is not considered to suppress the aforementioned non-uniform deformation which tends to occur when forming an aluminum alloy wire from an aluminum alloy wire rod by wire drawing and it is also not considered to improve impact resistance and bending fatigue resistance which are properties necessary for an aluminum alloy wire used under a severe operating environment in which a fatigue fracture is likely to occur due to repeated bending stress applied due to opening and closing of a door.